T. Nakatsuka

LAAS network observation of air showers

Abstract A network observation of air showers is in progress in Japan. Researchers from 10 institutions are taking part in the network, forming the Large Area Air Shower (LAAS) group. As of June 2000, eight air shower arrays (stations) are in operation, while five more stations are under construction. The stations are scattered over an enormous area of 130,000 km2, however the Global Positioning System (GPS) provides comparable time stamps among them with an accuracy of one microsecond. The aim of the network is to find out correlations in primary cosmic rays just below the knee. The data from the planned Norikura array (2,770 m a.s.l.) is expected to reveal new aspects of correlated cosmic rays. The current status of the network and performance of arrays calculated by the air shower simulation code CORSIKA are described here.

Search for coincident air showers in the network observation

Abstract Here we report results of the coincident event analysis, using air shower data taken at six stations of the Large Area Air Shower (LAAS) group in Japan. In this analysis we search for pair sof coincident air showers between stations, which are hypothetically induced by bursts of ultra-high-energy γ-ray sources or by secondary particles from interactions of extremely-high-energy cosmic rays with interstellar matter. From four years data (2.3 × 10 6 air showers) we find a pair of air showers with a very small time difference of 195 microseconds and an angular distance of 5.3 degrees, which is within the angular accuracy of our arrays, between two stations separated by 152 km. The chance probability of this event is 0.16. The Crab Nebula, a well-known ultra-high-energy γ-ray source, is within the angular accuracy from the arrival direction of this event. We expect that this event was induced by the ultra-high-energy γ-rays emitted in a burst of the Crab Nebula.

Network observation of air showers as cosmic ray interferometer

Abstract The Large Area Air Shower (LAAS) group has been performing a network observation of extensive air showers (EAS) since 1996. Ten compact EAS arrays (stations) are scattered over a large part of Japan and operated independently and simultaneously. Each station has 4–12 scintillation counters and a Global Positioning System (GPS), which provides time stamps of EAS arrivals with an accuracy of 1 μs. The primary purpose of the network observation is to study large-scale correlations in ultra-high-energy cosmic rays, which are attracting much interest of the astroparticle physicists community in recent years. The present state of the network and some results from computer simulations are reported here.

The LAAS network observation for studying time correlations in extensive air showers

The Large Area Air Shower (LAAS) group has been performing a network observation of extensive air showers (EAS) since 1996 in Japan. Ten compact EAS arrays are operating simultaneously at distant stations (up to ≈1000 km) and detecting EAS with mean energy of ≈1015 eV. Each station has 4--12 scintillation counters and a Global Positioning System (GPS), which provides time stamps of EAS triggers with an accuracy of 1μs. As a consequence of the comparable time stamps, uniformly-adjusted detectors and a standardized data format among all stations, we can treat the independent observations as a gigantic EAS detector system as a whole. The primary purpose of the network observation is to study large-scale correlations in ultra-high-energy cosmic rays. On the other hand, three nearby stations within 1~km distance at Okayama area have a possibility to detect extremely-high-energy EAS (≈1019 eV) as coincident triggers of the three stations. The present status of the network and some results from computer simulations are reported here.

Survival probability of charged particles dissipating their energies by radiation and ionization

The survival probabilities of charged particle traversing through matters are evaluated by solving the diffusion equation, taking account of radiation and ionization losses. The solutions corresponding to high and low incident energies are proposed respectively, and inaccuracies of the saddle point method appearing in case of large probabilities are removed by deriving the results via the complementary probabilities. The enough-accurate results will be valuable to improve the resolution of radiographic analyses with atmospheric muons, and others.

Update of the observation system for the primary energy spectrum with compact EAS arrays in LAAS experiment

The relation between the core distance and the time structure of extensive air shower (EAS) particles shown by Linsley provided to determine the primary energy by using compact EAS arrays. In an apparatus for restricting the zenith angle of EAS event, relocation and update of the apparatus was carried out. And the simulation for this upgrading was performed. We report the simulation results and comparisons of old and new apparatus.

The primary energy spectrum derived from Linsley method with simulations of heavy compositions in the LAAS mini array observation

The primary energy and its spectrum are obtained by using compact Extensive Air Shower (EAS) arrays, the apparatus for restricting the EAS zenith angle, and the Linsley method. In this work, we carried out the simulation and the data analysis by taking into account of proton and iron primaries. The primary spectral indices α(proton) and α(iron) are obtained as α(proton) = −2:47 ± 0:17 and α(iron) = −3:38 ± 0:25, respectively, in the single observation of a compact EAS array, in the primary energy region of 10 16 eV to 10 18:5 eV. Additionally, α(proton) = −3:12 ± 0:13 and α(iron) = −3:52 ± 0:17 are obtained with each composition in the synchronized observation between the compact EAS array and the apparatus for restricting the EAS zenith angle in the same primary energy region.

Simulation of electron trajectories in nuclear emulsion and its application

Atsushi Iyono∗a, Saya Yamamotob, Hiroki Matsumotoc, Kazuhide Okeic and Takao Nakatsukad, aDept. of Fundamental Science, Faculty of Science, Okayama university of Science 1-1 Ridai-cho, Kita-ku, Okayama-city, Okayama 700-0005, Japan bGraduate School of Science, Okayama University of Science 1-1 Ridai-cho, Kita-ku, Okayama-city, Okayama 700-0005, Japan cKawasaki Medical School 577 Matsushima, Kurashiki-city, Okayama 701-0192, Japan dOkayama Shoka University 2-10-1Tsushima Kyomachi, Kita-ku, Okayama-city ,Okayama 700-8601, Japan E-mail: iyono@das.ous.ac.jp,s15sm08@ous.jp,h.matsumoto@med.kawasaki-m.ac.jp, okei@med.kawasaki-m.ac.jp,nakatuka@olive.plala.or.jp

Simultaneous, parallel and multiple EAS events due to Gerasimova‐Zatsepin effect in LAAS experiments

To study the ultra‐high energy cosmic ray composition by making use of the GZ scenario, Large Area Air Shower (LAAS) experiments had been started in 1996 by the collaboration of 7 institutes in Japan. We present here some results of LAAS observations on the GZ event search during the data period from 1996 to January 2007. A brief profile of the potential of LAAS EAS array is also presented.

Observation of coincident air showers over a very large area

Abstract A network observation of air showers ( E 0 ≈ 10 15 eV) has been performed since 1996 in Japan. Ten compact air shower arrays, scattered over a large part of Japan, are operated independently by each institute. To explore large-scale correlations in primary cosmic rays, air shower data from five arrays of the network are analyzed in terms of arriving phases of coincident air showers, hypothetically induced by ultra-high-energy γ-rays from point sources or by secondary particles from interactions of extremely-high-energy cosmic rays with interstellar matter. Four coincident event candidates with extremely small time differences are found out. Two of these events are observed in the direction of the Crab Nebula, a well-known ultra-high-energy γ-ray source. However, significances of the events are not enough to invoke the claim for the detection of large-scale correlations.